Landscaping materials, such as mulch, are placed over a soil surface around trees and plants. Mulch is intended to improve soil conditions by retaining moisture within the soil, minimizing weed competition and distributing water to the soil. Mulch can also provide landscapes with a well-groomed appearance.
Mulch may be comprised of a plurality of fragments, such as chips, shredded bark, shells, etc. The fragments may be made from grinding, chopping, or otherwise reducing bulk material into the desired form. Two major types of landscaping mulch are inorganic and organic. Inorganic mulches include various types of stone, lava rock, pulverized rubber, geotextile fabrics and other materials. Inorganic mulch does not decompose and does not need to be replenished often. Because this type of mulch, does not decompose, it does not significantly improve soil structure, add organic materials or provide nutrients. Organic mulch is usually derived from plants and may include wood chips, pine needles, bark, cacao hulls, leaves, compost mixtures and a variety of other products. Organic mulches decompose in the landscape at different rates depending on the material and climate.
The marketability of mulch, particularly for decorative landscaping purposes, often depends on the appearance of the product and its color. Coloring agents or coatings have been used to artificially color landscaping material for a number of years. The coloring agents allow for greater control and uniformity in the appearance. The coloring agents are commonly sold in concentrated form and are mixed with water prior to their introduction to the landscaping material. The colorants used as part of the coating may also include admixers, such as resins and surfactants. U.S. Pat. No. 4,932,156 to Underwood describes a method for controlling the color of mulch that may become faded due to prolonged exposure to light and ambient weather conditions. The Underwood patent describes a composition generally comprised of a pigment or dye, a binding additive, glycol and water. This Underwood patent is herein incorporated by reference.
A number of machines exist for adding colorants to landscaping material. U.S. Pat. No. 6,551,401 to Winistorfer et al. describes a machine for coloring landscaping material having multiple mixing chambers. A first mixing chamber includes spraying of the coating material onto the mulch material and a second chamber wherein the coating is further applied in the form of an aerosol. Paddles supported by arms are secured to a rotating shaft within the mixing chambers to mix the landscaping material during the coating process. This patent is herein incorporated by reference.
Mulch provides many functions. To most effectively perform these functions, the mulch preferably provides a fairly uniform moisture level throughout the mulch bed. In addition, mulch should direct water evenly into the root system of the adjacent trees and plants, preventing the water from running off. For a few reasons, conventional organic mulch often does not create uniform moisture levels nor efficiently distribute water evenly over the soil bed.
The difference in the surface energy of organic mulch and water typically causes the water to “bead-up” or form droplets on the surface of the mulch material. Where the surface energy of a liquid is greater than the substrate on which the liquid is applied, the liquid is likely to bead-up on the surface of the substrate. Conversely, where the surface energy of the substrate is greater than the surface energy of the applied liquid, the liquid is more likely to “wet” or coat the surface of the substrate. Similarly, as the surface energies of the substrate and liquid align appropriately, the liquid begins to more uniformly and effectively wet the substrate. “Wetting” refers to a liquid forming a film-like coating with a substantially uniform thickness over the substrate rather than beading up or forming droplets. Wetting may also refer to a condition wherein droplets are more spread out or less spherical. In other words, the droplet has a contact angle, or angle at which a liquid interface meets the substrate's surface that is less steep and therefore forms a shallow profile on the surface of the substrate.
When the water beads-up on the surface of a substrate, the droplets coagulate on a portion of the surface, accumulate and then run-off. The run-off forms channels of water that move through a path of least resistance. In mulch materials, the path of least resistance is typically a path created by gaps in the landscaping material. Another path of least resistance is over the surface of the mulch material. Water that does not move through the gaps in the mulch material simply runs-off the surface of the bed, without being directed into the mulch bed or to the underlying soil bed. The random paths and surface run-off results in an uneven application of water to the soil, soaking certain areas and leaving large dry spots in other areas.
The surface energy of organic materials, such as wood chips, is generally much less than that of water. For example, the surface energy of oak chips without a colorant is roughly 35 dyne/cm at 25 degrees Celsius. The surface energy of water is roughly 72 dyne/cm at 25 degrees Celsius. Based on this differential between the surface energy, water will bead-up on the surface of the oak chips rather than wet the surface. In addition, organic landscaping material comprised of naturally-occurring substances has an irregular or non-uniform composition. Thus, some mulch chips in a batch may have different surface energies and different portions of individual chips may also have variations in surface energy. This also contributes to the uneven flow of water.